This invention relates to hydrodynamic tilt pad bearings for use in rotating machinery such as generators and turbines. More particularly, this invention relates to the lubrication and cooling of tilt pad thrust and journal bearings and to the manner in which axial loads are handled in tilt pad thrust bearings.
Tilting pad journal and thrust bearings are known for use with high speed rotating shafts and rotating collars. The tilting pads pivot to uniformly distribute the load imposed upon them by the rotating collar. Each of the tilt pads typically has a working surface made of babbitt metal that is lubricated by a fluid film.
One major concern with such bearings is that the pad working surfaces be kept below a predetermined temperature to prevent the rotating surface from wiping and to maximize the performance of the machinery. The typical way in which the bearing temperature is controlled is by using an excessive volume of cool lubricating fluid. In some prior art bearings, the pad working surfaces are substantially flooded with the lubricating fluid. A disadvantage of such so-called "flooded" bearings is that the amount of oil provided to each tilting pad cannot be controlled. This is a significant disadvantage in the case of journal bearings since the loads applied to the tilt pads in a journal bearing are typically unequal. Therefore, it is desirable to apply varying amounts of lubricating fluid to the tilt pads, based upon their respective loads.
Another disadvantage of flooded bearings is that they require a great deal of lubricating fluid, and corresponding apparatus for circulating substantial amounts of lubricating fluid. Another disadvantage of flooded bearings is that the lubricating fluid must find its way to the drain holes, without being directed towards the drain holes. Yet another disadvantage with flooded bearings is that bearing power losses increase due to the additional oil shear in the oil path to the pad.
Tilt pad thrust bearings are known in which the flow of the lubricating fluid is directed by a stationary lubricant director, as an alternative to simply allowing the fluid to find the exit. For example, U.S. Pat. No. 5,271,676 issued Dec. 21, 1993 discloses a combination journal/thrust bearing having lubricant directors between adjacent thrust tilt pads to direct the flow of lubricating fluid to a succeeding bearing pad in a direction of shaft rotation and to dispose of lubricating fluid from a preceding pad to an oil sump.
In a typical prior art thrust bearing, the lubricating fluid is introduced into the bearing housing, and then proceeds in an axial direction toward the rotating collar before it flows onto the tilting pads. Unfortunately, this path for the fluid causes the lubricating fluid to pick up heat as the fluid moves in the axial direction. The fluid also mixes with the hot carryover fluid from the preceding, adjacent pad. As a result, the temperature of the fluid when it reaches the leading surface of a tilting pad is substantially greater than the temperature of the fluid when the fluid entered the bearing housing. The resulting higher pad temperatures reduce the load carrying ability and the safety margin of the bearing assembly.
In typical equalized tilting pad thrust bearings, the tilting pads are supported by upper equalizing links which in turn are supported by lower equalizing links. The lower links are supported by a retainer. The tilting pad-link assembly is kept together by the retainer, which has an inner retainer ring and an outer retainer ring. Axial loads imposed upon the tilting pads are transferred through the upper and lower links to the retainer, which in turn is disposed within the bearing housing. The tilting pads typically have a spherical lower surface that makes point contact with an upper link so that the tilting pads may pivot in both the radial and the circumferential directions. Each upper link typically contacts two adjacent lower links on two spaced flat surfaces. The lower links in turn transfer the axial loads to the retainer.
There are several disadvantages to the prior art equalized bearing described above. First, the tilting pads tend to thermally and elastically deflect in the axial direction about their respective pivots under high axial loads, thereby reducing the effective bearing surfaces of the tilting pads. The load carrying ability and the bearing safety margin are reduced.
A second disadvantage of such prior art equalized tilting pad bearings is that the bearing has a high axial profile because of the number of components that are stacked onto each other. The tilting pads are stacked on the upper links, which in turn are stacked on the lower links. The lower links rest on the retainer, which in turn is enclosed by the bearing housing. The height of a typical equalized thrust bearing (with a 10.5" O.D.) is about 3.38 inches.